Radio signals are subject to several propagation phenomena that greatly impact on the strength of the signal at a receiver. If the signal reflects from buildings or other man-made or natural surfaces, the reflected signal, which follows a different, longer path than a direct signal, can arrive at the receiver out of phase with the direct signal; the resulting interference between the two or more signals reduces the signal strength at the receiver. This phenomenon is multi-path interference.
A related type of interference arises in simulcast paging systems wherein the same signal is broadcast from a plurality of transmitters. A receiver disposed in a zone where identical transmitted signals from two or more transmitting stations overlap can experience a reduction in the strength of the overall received signal, due to a destructive interference that occurs when the radio signals received from the different transmitters are summed together, since the propagation distance, and therefore, the phase of the signals can be quite different. Simulcast interference is thus similar in its effect, but differs from multi-path interference in that the former involves interference between signals from different transmitters, while the latter involves interference between signals from the same transmitter that travel along different propagation paths. In addition, since it is virtually impossible to precisely set the transmit center RF carder frequencies of the overlapping transmissions, the receiver in a simulcast paging system experiences an additional distortion problem that is not evident in multi-path interference of signals transmitted from a single transmitter. In flat fading, the frequency response of the received signal is flat and only the gain and phase fluctuate.
Flat fading represents yet another phenomenon by which received signal strength can be diminished. However, when flat fading occurs, only a signal from one transmitter is involved, and this signal is propagated without reflection, directly to the receiver.
To mitigate the effects of multi-path interference at the receiver, multi-level low baud rate transmissions are sometimes employed. This technique tries to minimize the effect of multi-path distortion by making the relative time/phase difference in the received multi-path signals relatively small compared to the data baud rate. The most significant drawback of this approach is its limitation on data rate, which causes poor system performance. Further, the bit error rate (BER) of the received signal suffers, even when a relatively strong received signal is available, and the modulation of a signal at multiple levels is complicated by the need to split the signal into different sub-bands, which increases processor loading. Use of multi-level, low baud rate modulation techniques is discussed in "MTEL Petition for Rule Making to Allocate Frequencies for New Nationwide Wireless Network Services," a petition submitted to the FCC on Nov. 12, 1991.
Another technique applied to reduce the effect of multi-path interference is the use of equalizers, including fast Kalman equalizers that attempt to rapidly track and converge on dynamically changing conditions. However, this technique has previously not been proposed for use in a simulcast system. Conventional equalizers are relatively slow in adapting to distortion, causing a relatively high BER until the equalizer properly converges. Although faster Kalman equalizers can reduce this problem, the algorithm employed in the devices is so computationally complicated that it requires significant processing overhead. Also, the algorithm employed in Kalman equalizers is somewhat unstable. In fact, when rapid fading occurs due to multi-path distortion, equalizers tend to become unstable and are more likely to fail. A textbook entitled Digital Communications, 2nd edition, by J. Proakis, published by McGraw Hill in 1989, describes the use of equalizers for this purpose in Chapter 6, pages 519 through 693.
Use of pilot symbol assisted modulation (PSAM) is a technique well known in the prior art for minimizing the effect of flat fading, and is particularly effective for use with mobile receivers. Several references describe how BER caused by flat fading can be substantially reduced using pilot symbols, including, for example, "An Analysis of Pilot Symbol Assisted Modulation for Rayleigh Fading Channels," by J. K. Cavers, in IEEE Transactions on Vehicular Technology, Vol. 40, No. 4, November 1991, and "TCMP--A Modulation and Coding Strategy for Rician Fading Channels," IEEE Journal on Selected Areas of Communications, L. Moher and J. H. Lodge, Vol. 7, pp. 1347-1355, December 1989. This technique uses a single pilot symbol in each frame transmitted. These references only discuss the use of pilot symbols in connection with reducing the fading of a single ray following a direct path from a single transmitter site and do not teach or suggest how pilot symbols might be used to deal with multi-path or simulcast interference problems.
A paper entitled "Adaptive Equalization and Diversity Combining for a Mobile Radio Channel," by N. Lo, D. Falconer, and A. Sheikh, Proc. IEEE Globecom '90, December 1990, discloses a digital cellular radio system, which employs a jointly adaptive decision-feedback equalizer and diversity combiner to mitigate Doppler fading rates up to 100 Hz. As disclosed in this reference, current estimates of channel impulse response are interpolated and the interpolated values applied to successive data symbols in blocks of data, to compensate for changes in the channel impulse response with time. The technique provides for transmitting a plurality of predefined pilot symbols before each block of data and interpolating the estimated channel impulse response for the symbols received before and after each block of data. The interpolation attempts to minimize the effect of relatively fast Doppler fading on the data. However, this approach achieves only a modest performance gain, because it fails to consider channel conditions, such as relative signal strength of the received signals, the Doppler fading frequency, propagation delay differences between the interfering signals, frequency offsets between the interfering signals, and the signal-to-noise ratio of the received signals, when carrying out the interpolation process. Consequently, the method disclosed in this paper can not provide an optimal compensation under extreme signal fading conditions for the channel.
Accordingly, it should be evident that compensation for multi-path and simulcast interference is required that enables high-speed data transfer. The prior art systems are either limited to low data rates, have too high a BER, require too much processing overhead, or fail to provide optimal mitigation of the fading problem.